Optical elements may be coated with thin dielectric films to protect them and/or to reduce surface reflections. Born, M. and Wolf, E., Principles of Optics, p. 51, Pergamon Press, New York (1970. Armstrong, K. R. and Low, F. J., Appl. Opt. 13, 425 (1975), describe a technique (useful for wavelengths greater than 20 .mu.m) for using thermally-bonded polyethylene and Mylar (Du Pont De Nemours & Co., Wilmington, DE 19898) films as antireflection coatings on the surfaces of transmitting materials. The indices of refraction of polyethylene (n.about.1.5) and Mylar (n.about.1.7) make them well-suited for this purpose, and they form thermal bonds with many materials which will survive repeted cryogenic cycling below 4K.
Although these films adhere well to optical elements, it is difficult to obtain a smooth, flawless surface when using them, especially for diameters greater than 1 cm. The imperfections come from wrinkles in the plastic and from residual air bubbles.
Other optical elements comprising a single thin sheet of dielectric film mounted on an annular ring or clamped in a holder, such as a pellicle or beam splitter, must also be free of such imperfections.